The present invention relates to a kit for the production of bone cement, a paste for the production of bone cement, and uses of the kit and the paste.
Bone cements based on poly(methyl methacrylate) (PMMA) have been known for decades and trace back to the basic work done by Sir Charnley (Charnley, J., “Anchorage of the femoral head prosthesis of the shaft of the femur,” J. Bone Joint Surg. 42: 28-30 (1960)). In principle, the basic configuration of the PMMA bone cements has remained the same since then. PMMA bone cements comprise a fluid monomer component and a powder component. The monomer component includes, in general, the monomer methyl methacrylate and an activator (for example, N,N-dimethyl-p-toluidine) dissolved in this monomer. The powder component comprises one or more polymers produced by polymerization, preferably suspension polymerization, based on methyl methacrylate and comonomers, such as styrene, methyl acrylate, or similar monomers, a radiopaque material, and the initiator (for example, dibenzoyl peroxide). When the powder component is mixed with the monomer component, due to the swelling of the polymers of the powder component in the methyl methacrylate, a plastically deformable paste is produced. Simultaneously, the activator N,N-dimethyl-p-toluidine reacts with the dibenzoyl peroxide, which breaks down with the formation of radicals. The formed radicals initiate the radical polymerization of the methyl methacrylate. With advancing polymerization of the methyl methacrylate, the viscosity of the cement paste increases until the paste solidifies and is thus cured.
The basic mechanical requirements on PMMA bone cements, such as 4-point flexural strength, flexural modulus, and compression strength, are described in ISO 5833. For the person applying the PMMA bone cement, the property of non-adhesiveness of the bone cement is of significant importance. The term “non-adhesiveness” is defined in ISO 5833. For conventional PMMA bone cements, non-adhesiveness indicates that the cement has reached the workable phase after the mixing of the components due to the swelling of the polymers contained in the cement powder in the monomer. Basically, a PMMA bone cement must be non-adhesive, so that the user can form and apply the cement. The PMMA bone cement must not adhere to gloves and application aids, such as mixing systems, crucibles, or spatulas.
The significant disadvantage of the previous PMMA bone cements for the medical user consists in that the user must mix the liquid monomer component with the powder component in a mixing system or in crucibles directly before the application of the cement. Here, mixing errors can easily occur, which could negatively affect the cement quality. The mixing must be performed in an uninterrupted process. Here, it is important that the entire cement powder be mixed with the monomer component without the formation of clumps and that during the mixing process the entry of air bubbles be avoided. With the use of vacuum mixing systems, in contrast to hand mixing, the formation of air bubbles in the cement paste is largely prevented, but an additional vacuum pump is required for these systems. Examples of mixing systems are disclosed in the publications: U.S. Pat. No. 4,015,945, European patent application publication EP 0 674 888 A1, and Japanese patent application publication (Kokai) JP 2003/181270 A. Vacuum mixing systems and vacuum pumps are relatively expensive. After the mixing of the monomer component with the powder component, depending on the type of the cement, a more or less long time must elapse until the cement paste is non-adhesive and can be applied. Due to the many possible errors in the mixing of conventional PMMA bone cements, appropriately trained personnel are needed. The training entails not insignificant costs. Furthermore, the mixing of the fluid monomer component with the powder component leads to an exposure of the user to monomer vapors and to the release of powdery cement particles.
In order to prevent these conditions during the production of a bone cement from a fluid monomer solution and a polymer powder, German Patent DE 10 2007 050 762 B3 proposes a paste-like bone cement. This paste-like bone cement is based on the idea of dissolving a polymer in a methacrylate monomer and suspending in this solution a particulate polymer not soluble in the methacrylate monomer. In this way, it is possible to produce a paste-like mass that exhibits high inner cohesion due to the dissolved polymer and has high viscosity due to the particulate, non-soluble polymer, so that the paste can temporarily withstand the bleeding pressure. Due to radical polymerization of the methacrylate monomers, the paste can be cured. The radical polymerization is possible with (i) a radical initiator, such as barbituric acid derivatives or dibenzoyl peroxide, and (ii) a copper salt as the activator. It has been shown, however, that with the use of this initiator system, the formed bone-cement paste does not cure uniformly, but instead from the core outward in the direction of the surface of the formed bone-cement paste. Due to the evaporation of the monomer contained in the bone-cement paste, bubbles are formed in the resulting bone cement. Furthermore, it has been observed that with the use of this initiator system, the monomers contained in the bone-cement paste are not completely converted. These circumstances have a disadvantageous effect on the physical properties of the bone cement, in particular on the impact strength.